Low impedance slotted line



July 2, 1968 R. A. FELSENHELD LOW IMPEDANCE SLOTTED LINE Filed Oct. 22, 1965 INVENTOR.

ROfl'RT A. FELSENHELD l m mm ATTORNEY United States Patent 3,391,355 LOW IMPEDANCE SLOTTED LINE Robert A. Felsenheld, Livingston, N.J., assignor to International Telephone and Telegraph Corporation, a corporation of Delaware Filed Oct. 22, 1965, Ser. No. 500,830 8 Claims. (Cl. 333-31) This invention relates to slotted transmission lines for high and low frequency measurement of impedances and the like, and more particularly to a slotted line having a helically wound conductor.

Slotted transmission lines for measuring impedances and the like having straight inner conductors have long been known in the art. For example, to measure an impedance at a given frequency one must measure the maximum and minimum value of the signal radiated from a slotted line driven with a signal of predetermined frequency and terminated by said impedance. Also the distance from the terminating load to the first minimum must be measured. Th se maxima and minima will occur one-quarter wavelength apart. Since it is not known how far from the end of the line the first minimum of the radiated signal will occur, a slotted line whose length is approximately onehalf wavelength is required. At low frequencies the length of a line having a straight inner conductor becomes very large and impractical to build. Therefore a line with a helical inner conductor, the uncoiled length thereof being approximately equal to one-half wavelength, could be utilized to shorten the physical length of said line for a given electrical length.

The use of such a helical inner conductor now causes the characteristic impedance of the slotted line to increase since the helically coiled inner conductor increases the inductance of the line. The effect of this increased inductance on the characteristic impedance is readily seen by examining the equation for the characteristic impedance of a transmission line, to wit:

where L is the inductance of the line and C is the capacitance of the line. This increases in the characteristic impedance is undesirable since it causes a mismatch with the driving and terminating equipment, thereby producing unwanted standing waves and inaccuracies in the resulting measurements.

Therefore, it is the object of this invention to provide a slotted line having a helical inner conductor which has a characteristic impedance in the same order of magnitude as the characteristic impedance of a slotted line having a straight inner conductor which operates in the same frequency range.

According to this invention a transmission line having a slotted outer conductor and a helically wound inner conductor is further provided with a third conductor mounted within said helical conductor and separated therefrom by a layer of dielectric. This third conductor is electrically coupled at one end to one end of the outer conductor of said line. The inclusion of the third conductor increases the capacity of the line, thereby reducing the characteristic impedance thereof according to the equation thus compensating for the increase in inductance introduced by said helically wound conductor.

The above-mentioned and other objects and features of this invention will become apparent by reference to the following description taken in conjunction with the following drawings, in which:

3,391,355 Patented July 2, 1968 "Ice FIGURE 1 s a cross-sectional view taken along the longitudinal axis of a slotted line according to the invention;

FIGURE 2 is a cross-sectional view taken perpendicular to the longitudinal axis of a slotted line according to this invention;

FIGURE 3 is a schematic illustration of a slotted line according to this invention utilizing a probe slidably mounted in the slot to detect the radiated field; and

FIGURE 4 is a schematic illustration of a slotted line according to this invention utilizing a current loop probe slidably mounted on said line to detect the radiated field.

As shown in FIGURES 1 and 2, one embodiment of this invention comprises a tubular outer conductor 1 having a slot 7 formed along its length. Mounted within said tubular outer conductor 1 is a helically wound inner conductor 3 whose length is such as to provide a line having the desired electrical length. Separating the helically wound conductor 3 from said slotted outer conductor 1 is a first layer of dielectric 2. This dielectric 2 usually would be air, but it is apparent that any other suitable dielectric may be used. Mounted within the helically Wound inner conductor 3 is a third conductor 5 which is spaced from said helical conductor 3 by another layer of dielectric 4. This dielectric could be air, but it is apparent that any other suitable dielectric may be used. The third conductor 5 is connected at one end to one end of the slotted outer conductor 1 by means of conductor 6, for example. This is to maintain them at the same relative potential without producing current loops. The insertion of the third conductor 5 within said helically wound inner conductor 3 serves to increase the capacitance of the line, thereby compensating for the increased inductance introduced by the helically wound conductor 3. This decreases the characteristic impedance of the slotted line-according to the equation to a value consistent with the characteristic impedance of a slotted line operating in the same frequency range but having a straight instead of helically wound inner conductor. It should be noted that ideally, all of the conductors should be mounted substantially concentrically within each other.

In FIGURE 3 there is shown a slotted line according to the invention utilizing a slidably mounted probe 8 for detecting the electric field radiated from said line. This type of probe is inserted in the slot 7 and is commonly used in known slotted lines having straight inner conductors. Due to the construction of the subject slotted line (i.e., the inner conductors are helically Wound) undesirable peaks and valleys in the radiation pattern will occur along the length thereof since the probe 8 will be in varying degrees of proximity to the helically wound inner conductor 3 as it is moved along said line. For accurate measuren ents using this type of probe, it would be most desirable to only measure the radiated field at the points where the helically woundinner conductor is closest to the probe. Otherwise, the peaks and valleys in the radiation pattern must be taken into account in the measurernents.

In order to eliminate the above-mentioned disadvantage of the probe 8 shown in FIGURE 3, the probe 9 of FIG- URE 4 may be utilized. In this embodiment, a current loop 9 is slidably mounted on said slotted line to detect the magnetic field radiated therefrom. By utilizing a current loop 9 for detecting the radiated field, the response of the line is effectively smoothed out. That is, the small undulations (peaks and valleys) that appear while utilizing the probe 8 of FIGURE 3 do not appear while utilizing the probe 9 of FIGURE 4. This is because the current loop probe is never really in very close proximity to the helically wound inner conductor 3, and due to fringing effects of the radiated field, the said field is eifectively smoothed out. For measuring impedances utilizing the probe 9 of FIGURE 4 it is now necessary to measure the maximum and minimum currents and the distance of the current minimum from the end of the line. While I have described above the principles of my in vention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention, as set forth in the accompanying claims.

I claim:

1. A low impedance slotted line of predetermined length comprising:

a hollow outer conductor having a slot formed along the length thereof;

a helically Wound inner conductor mounted within said outer conductor;

a first layer of dielectric between said outer conductor and said helical inner conductor;

a third conductor mounted within said helical inner conductor, one end thereof being electrically coupled to one end of said outer conductor;

a second layer of dielectric between said third conductor and said helical inner conductor; and

means coupled to said line for detecting the field radiated therefrom at various points along its length.

2. A slotted line according toclaim 1 wherein said means for detecting includes:

a probe slidably mounted within said slot; and

means for moving said probe along the length of said line.

3. A slotted line according to claim 1 wherein said means for detecting includes:

means forming a current loop slidably mounted around i said outer conductor for detecting the radiated field; and

means for moving said current loop along the length of said line to detect the strength of said radiated field at various points along said line.

4. A slotted line according to claim 1 wherein the electrical length thereof is approximately one-half wavelength.

-5. A slotted line according to claim 1 wherein one end of said third conductor is directly connected to one end of said outer conductor.

6'. A slotted line according to claim 1 wherein said outer conductor has a cylindrical cross-section.

7. A slotted line according to claim 1 wherein said third conductor is hollow.

8. A slotted line according to claim 7 wherein said third conductor has a cylindrical cross-section.

References Cited UNITED STATES PATENTS 2,843,791 7/1958 Pierce 333-31 2,915,718 12/1959 Grieg et al. 333-84- 3,l99,054 8/1965 Holland et al. 33384 OTHER REFERENCES Generation and Transmission of Microwave Energy, Dept. of the Army, Technical Manual TM 1l-6 73, G.P.O., Washington, DC. 1953.

Harvey, A.F., Microwave Engineering, Academic Press, New York, 1963.

ELI LIEBERMAN, Primary Examiner. HERMAN KARL SAALBACH, Examiner.

L. ALLAHUT, Assistant Examiner. 

1. A LOW IMPEDANCE SLOTTED LINE OF PREDETERMINED LENGTH COMPRISING: A HOLLOW OUTER CONDUCTOR HAVING A SLOT FORMED ALONG THE LENGTH THEREOF; A HELICALLY WOUND INNER CONDUCTOR MOUNTED WITHIN SAID OUTER CONDUCTOR; A FIRST LAYER OF DIELECTRIC BETWEEN SAID OUTER CONDUCTOR AND SAID HELICAL INNER CONDUCTOR; A THIRD CONDUCTOR MOUNTED WITHIN SAID HELICAL INNER CONDUCTOR, ONE END THEREOF BEING ELECTRICALLY COUPLED TO ONE END OF SAID OUTER CONDUCTOR; A SECOND LAYER OF DIELECTRIC BETWEEN SAID THIRD CONDUCTOR AND SAID HELICAL INNER CONDUCTOR; AND MEANS COUPLED TO SAID LINE FOR DETECTING THE FIELD RADIATED THEREFROM AT VARIOUS POINTS ALONG ITS LENGTH. 